Pigment particles and other dispersed solid particles usually have the tendency to flocculate and/or to precipitate. A dispersing agent prevents this flocculation and as a result stabilizes the dispersion. In addition, a dispersing agent lowers the viscosity of the dispersion. As a result the dispersion can be pumped or processed more easily at higher concentrations of solid particles.
In papermaking processes mineral fillers or pigments are usually introduced in the form of a dispersed filler/pigment. Useful dispersing agents are following: lignosulphonates such as Na lignosulphonate, condensation products of aromatic sulphonic acids with formaldehyde such as the condensed naphthalene sulphonates, dispersing anionic polymers, and copolymers made from anionic monomers or made anionic after polymerization, polymers containing repeating units having anionic charge such as carboxylic and sulphonic acids, their salts and combinations thereof. Also phosphates, non-ionic and cationic polymers, polysaccharides and surfactants may be used.
Inorganic dispersing agents have the disadvantage that they are not stable to hydrolysis, and some of the organic dispersing agents have the disadvantage of not being very effective.
There is a continuing need to develop more efficient dispersing agents.
WO 95/25146 discloses the use of an anionic polyelectrolyte as a dispersing agent for inorganic material, such as kaolin clay or calcium carbonate. The anionic polyelectrolyte dispersing agent may be a water soluble salt of poly(acrylic acid) or of poly(methacrylic acid) having a weight average molecular weight of less than 20000.
GB 1 593 582 discloses a viscosity-stable aqueous dispersion suitable for use as a paper coating composition, including a filler or pigment, and 0.05 to 5.0% by weight, based on the weight of the filler or pigment, of a dispersing agent which comprises a maleic anhydride copolymer, such as a copolymer of propylene and maleic anhydride or a copolymer of propylene and maleic acid-half amide.
Chang et al. reports in Macromolecules, Vol. 26, No. 18, 1993, pages 4814-4817, the synthesis and characterization of a copolymer of maleic anhydride and N-vinyl formamide. MA and NVF were copolymerized by free-radical polymerization in tetrahydrofuran or ethyl acetate.
U.S. Pat. No. 4,444,667 discloses a flocculant for sludges comprising a polymer prepared by homopolymerizing N-vinylformamide in the presence of a free-radical initiator to give a poly-N-vinylformamide, followed by hydrolysis of from 10 to 90% of the formyl groups in the poly-N-vinylformamide. Flocculants have the reversed effect as compared to dispersing agents which are typically deflocculants.
U.S. Pat. No. 4,126,549 discloses a process for the prevention of scale and the inhibition of corrosion which comprises adding to water a hydrolyzed terpolymer of maleic anhydride with vinyl acetate and ethyl acrylate. The molecular weight of the terpolymer is below 1000 U.S. Pat. No. 4,126,549 also discloses some hydrolyzed copolymers of maleic anhydride with e.g. vinyl acetate, acrylonitrile or ethyl acrylate. These terpolymers and copolymers are produced by polymerization in a solvent using free radical initiator at temperatures of 110°-113° C. In the copolymers, the molar ratio of maleic anhydride to the comonomer is at least 3:1.